To inject fuel into a combustion chamber, such as a cylinder, a fuel injector may be used, such as for example a solenoid valve or a solenoid injector. Such a solenoid injector (also called a coil injector) has a coil which generates a magnetic field when current flows through the coil, whereby a magnetic force is exerted on an armature so that the armature moves in order to cause an opening or closing of a nozzle needle or a closing element, for opening or closing the solenoid valve. If the solenoid valve or injector has a so-called idle stroke between the armature and the nozzle needle, or between the armature and the closing element, a movement of the armature does not lead to a movement of the closing element or nozzle needle immediately, but only after the armature has completed its movement through the idle stroke.
When a voltage is applied to the coil of the solenoid valve, electromagnetic forces move the armature in the direction of a pole piece or pole shoe. After overcoming the idle stroke, a mechanical coupling (e.g. a mechanical contact) also causes the movement of the nozzle needle or closing element and, for a corresponding shift, opens injection holes for the supply of fuel into the combustion chamber. If further current flows through the coil, the armature and nozzle needle or closing element move further until the armature reaches or stops on the pole piece. The distance between the stop of the armature on a carrier of the closing element or nozzle needle, and the stop of the armature on the pole piece, is also called the needle stroke or working stroke. In order to close the fuel injector, the exciter voltage applied to the coil is switched off and the coil is closed briefly so that the magnetic force discharges. The coil short-circuit causes a reversal of polarity of the voltage because of the discharge of the magnetic field stored in the coil. The level of the voltage is limited by a diode. The nozzle needle or closing element, including armature, is moved to the closing position because of a return force provided for example by a spring. The idle stroke and the needle stroke are run in reverse order. For short injection times, the closing process begins even before the armature has stopped on the pole piece, so the needle movement thus describes a ballistic trajectory.
The time of starting the needle movement on opening of the fuel injector (also known as OPP1) corresponds to the start of the injection, and the time of ending the needle movement on closing of the fuel injector (also known as OPP4) corresponds to the end of the injection. These two times therefore determine the hydraulic duration of the injection. Consequently, for identical electrical actuation, injector-specific temporal variations for the start of the needle movement (opening) and the end of the needle movement (closing) can lead to different injection quantities.
Accordingly, the above-mentioned times (and further relevant times) which correspond to specific opening states can be determined in various ways. These times are normally determined based on the coupling, caused by eddy current, between the mechanical elements (armature and injector needle) and the magnetic circuit (coil), which generates a feedback signal based on the movement of the mechanical elements. A speed-dependent eddy current is induced in the armature because of the movement of the nozzle needle and armature, which also causes a feedback on the electromagnetic circuit. Depending on the movement speed, a voltage is induced in the solenoid which is superposed on the actuation signal. Utilisation of this effect means that the superposition of the electrical base parameters of voltage or current with the signal change from the needle movement can suitably be separated and then processed further. The characteristic signal form in the voltage or current signal is evaluated relative to the time of occurrence. The signals for needle opening are then detected in the current profile, and those for needle closing in the voltage profile. This is illustrated as an example for OPP1. The armature meets the needle and carries this with it. The change in armature speed induces an eddy current which can be detected in the current profile as the OPP1 signal.
Thus for example, the time OPP1 at which the armature meets the needle and carries this along, whereby the change in armature speed induces an eddy current and the needle movement begins, can be determined by determining a local maximum in the second temporal derivative of the coil current. However, there is the occurrence of several maxima, so an additional plausibility check process is necessary in order to determine the correct maximum.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.